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Old 05-13-2004, 09:14 AM   #15
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Originally Posted by markdoane
Is it possible (if money were no object) to put a small tank heater downstream from the tankless heater? I'm thinking this would overcome a frequent complaint about tankless systems, which is the time required to fire up and reach temperature. I think that would overcome the problem of water wastage when all you need is a quart of hot water to wash a few dishes.

It is possible with the right amount of money, but...

Water wasting waiting for it to get hot is a function of distance from the heating appliance to the faucet, volume of the water pipe, temperature of the captured water in that pipe, and the rise time of the heating device.
The way I see it you must over come these obstacles.

The best way to do this is shorten the run from the heater and speed up the temperature rise time.
This can be done by placing a tank type water heater right at the faucet but you just gave away the advantage of a tankless heater.
Give and take.
You might look at a small electric tankless heater (Point of Delivery), I have seen them in plumbing stores and could be adapted to take care of your problem.
The only problem I see is you could only use it when hooked up to shore power.
Give and take.
I am not aware of any small gas powered under sink units.
Good luck with your quest.

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Old 05-13-2004, 09:22 AM   #16
Rivet Master
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Reno , Nevada
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I think you are going to get hot water from a tankless just as fast as you would from a tank. Either way, the problem is the water currently in the lines. One of the more interesting ideas I have seen for this problem was credited to Trailer Life and suggests putting a valve in the hot water line that will release water back to the supply tank. Then you can purge the cool water in the hot water lines without wasting water.

A tankless water heater that requires time to fire up and reach temperature would be pretty much useless. Avoiding this lack of utility is why their burners are 4x more powerful than those on a tank type, why they don't store water, and why the flow sensor is important.

For the water lines in an RV, figure about 44 feet of line per quart of water. This is 2 lb. From the input side of the tankless to a shower might be half this length or 1 lb of water. 40k BTU/hr (RV-500 output) is 11 BTU/sec and flow is likely to be about 10 lb/min or 1 lb in about 6 seconds. (please check my math before taking this too seriously!)

I read this as indicating it will take about 6 seconds to purge the hot water line and this would allow a tankless heater like the RV-500 to apply a 70F temp rise to the water going through it in this time.

I don't think the ignition lag in a tankless is going to be of much significance in this scenario as that is only a fraction of a second plus a second or two to transfer the heat.

Usually, people use hot water tanks and then place tankless heaters at the point of use as a means to avoid line purge wastage, not the other way around. You can find a lot of 'under the sink' low rate tankless water heaters in home improvement stores.

Oh, and by the way, Note that these electric under the sink tankless things are about 5,000 BTU/hr at most as that is a typical 15 Amp circuit.

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